8.27 shows the horizontal canals and utricle, as seen from above as the head is rotated (accelerated) to the left. As acceleration to the left begins, the inertia of the endolymph in the horizontal canals increases pressure toward the right side. This causes the cilia to bend on hair cells of the ampulla of the left horizontal canal toward the utricle and bends the cilia of the right canal away from the utricle. These actions increase the firing rate in the afferent fibers on the left and decrease the firing rate of the afferent fibers on the right. Once the head is moving at a constant velocity of rotation (i.e., no acceleration), there would be no force on either cupula, and therefore the hair cells of both canals would be firing as they do at rest. However, when the indicated rotation is stopped, the inertia of the endolymph creates a force on both cupulas, but in the direction opposite to that caused by the original acceleration. This results in an increase in the discharge rate of afferent fibers on the right side and a decrease in the discharge rate on the left. This postrotatory effect is of functional and clinical significance.
Unlike the hair cells in the ampullary crests, not all the hair cells in the otolith organs are oriented in the same •Fig. 8.28 Functional Polarization of Hair Cells in the Otolith Organs. A, Thesaccule.B, Theutricle.Thestriolaineachcaseisindicatedbythedotted line. (RedrawnfromSpoendlinHH.In:WolfsonRJ[ed].The Vestibular System and Its Diseases. Philadelphia:UniversityofPennsylvaniaPress;1966.)
Irritationofthevestibularlabyrinth,asinMeniere’s disease, canresultinrhythmicconjugatedeviationsoftheeyes,followedbyquickreturnsaccades.Thisconditionisknownasnystagmus (see
Chapter9).Theseeye movementsareaccompaniedbyasenseofvertigo andoftennausea. Thebraininterpretsadifferenceintheinputfromthetwosidesofthevestibularsystemasheadmotion.Irritation(ordestruction)ofonelabyrinthproducesanasymmetryofinputthatresultsinabnormaleyemovementandassociatedpsychologicaleffects.
direction. Instead, they are oriented in relation to a ridge, called the striola, along the otolith organ (see
Fig. 8.25 ). In the utricle, the hair cells on either side of the striola are polarized toward the striola, whereas in the saccule they are polarized away from the striola. Because the striola in each otolith organ is curved, there are hair cells with all orientations in the plane of the organ (
Fig. 8.28 ). In any particular orientation of the head, the cilia of the hair cells are bent to varying extents according to their orientation in relation to the gravitational vector. This results in a particular pattern of input from the otolith organs to the CNS. When the head is tilted to a new position, the orientation of the otolithic membranes in relation to the gravitational vector changes, and so the cilia of the hair cells are bent in a new way. This change in the bending of the cilia of the hair cells changes the pattern of input from the otolith organs to the CNS and creates the sensation of movement, as well as possibly triggering various reflexes. Similarly, a linear acceleration caused by other forces, such as might occur in a fall or the angular acceleration when a car turns around a curve (angular accelerations have linear centripetal and instantaneous tangential components), also affects output from the otolith organs.
The vestibular afferent fibers project to the brainstem through the vestibular nerve. As previously mentioned, the cell bodies of these afferent fibers are located in the Scarpa ganglion. These primary afferent fibers terminate in the vestibular nuclei (see
Fig. 4.6
D to E), which are located in the rostral medulla and caudal pons and in specific regions of the cerebellum, most prominently in the nodulus.